1. Field of the Invention
This invention relates to residential fuel oil reporting systems in which a each of a plurality of remote fuel oil level sensors monitors the fuel oil level in a particular oil tank and sends information to a central location in response to a low fuel oil condition.
2. Description of the Related Art
Fuel oil which is used to heat residences and buildings is generally stored in small tanks located either inside the building, outside the building or in the ground near the building. In order to ensure that an adequate supply of fuel is available to the building furnace, each residential tank must be periodically refilled by making a fuel oil delivery to the tank location. Such fuel oil deliveries are presently made by a central distributor utilizing small tank trucks.
A problem exists in this supply system in that the oil tanks must be filled before the supply of fuel runs out, but predicting the rate of consumption is difficult. It is not economical for the fuel oil distributor to refill the tanks on a set schedule, especially during the warmer seasons in which fuel oil consumption is at a minimum. Accordingly, fuel oil distributors may schedule a delivery to a particular tank based on a usage history for that tank and recent weather conditions. This type of delivery is referred to as a "degree-day" system, and uses mathematical algorithms to predict the rate of fuel oil consumption for each tank.
Because of variations in usage patterns from one location to another, predictive algorithms for determining when to make a delivery tend to be imprecise. For this reason, a number of prior art systems have been used which automatically monitor the level of oil in a tank, and communicate information regarding the oil level to a central receiving station, generally by means of a direct dial telephone network. In the central receiving station, the information is processed and a report is generated so that a delivery of fuel oil can be scheduled to replenish the customer's supply before it runs out.
Two types of remote signaling have been used in the past. In a first type, the central monitoring location initiates a telephone call to each remote location in order to gather tank level information. The telephone calls placed by these systems, however, can prove annoying to residents of the building where the tank is located, particularly if the calls come at an inconvenient time. An example of such systems is shown in U.S. Pat. Nos. 3,899,639 and 4,147,893.
In the second type, a sensor/signaling unit at each remote location monitors the fluid level in the oil tank and initiates a telephone call to the central location when a low fuel condition is detected in the tank which it monitors. These systems may include a means for testing the status of the phone line (which is typically also used by the residents for normal telephone service) so that the line is seized by the device only when there is no existing call detected on the line. Examples of such systems are shown in U.S. Pat. Nos. 3,588,357, 3,842,208, 4,059,727 and 4,486,625.
Because it is not uncommon for the monitoring system of a single distributor to include several hundred remote units, it is desirable to make each of the remote units as inexpensive and simple to install as possible. Some methods of reducing the overall cost of the remote sensing units include use of an inexpensive fuel level sensor, an inexpensive mechanism to transmit information to the central location over the telephone network and a construction which allows the unit to be easily and quickly installed.
These problems were addressed to some degree by U.S. Patent No. 4,845,486. This patent teaches a manner of encoding information for transmission by the remote unit which does not require an expensive, highly stable oscillator. It also teaches the use of a battery so that no external power supply connections are required. Furthermore, this patent describes the use of a reed switch within a sensor used by the remote unit. The sensor clips to a gauge common in many residential fuel oil tanks, and the reed switch is activated a magnet positioned on a level indicator which moves within the gauge when the oil level changes.